Understanding the multiscale self-assembly of metal-organic polyhedra towards functionally graded porous gels

Special Issue "Design, Synthesis and Applications of Macroporous, Mesoporous, and Microporous Materials"

The intention of this Special Issue was to highlight the importance of the design, synthesis, and applications of macro-, meso-, and microporous materials [...].

Coalescence of Porous Coordination Cages into Crystalline and Amorphous Bulk Solids

Discrete porous coordination cages are attractive as a solution processable material whose porosity is not predicated on a network structure. Here, we leverage the peripheral functionalization of these cage structures to obtain 12 novel, solution processable, porous coordination cages that afford crystalline and amorphous single-phase millimeter-scale monolithic bulk structures (six of each) upon solidification. These structures are based upon prototypal metal-organic polyhedra [Cu24(5-x-isophthalate)24] (where x = NH2, OH), wherein meta-substitution of linker ligands with acyl chloride or isocyanate moieties afforded amide and urethane functional groups, respectively. These porous cage structures were obtainable via direct synthesis between a metal salt and a ligand as well as postsynthetic modification of the cage and formed monoliths following centrifugation and drying of the product. We rationalize their self-assembly as colloidal packing of nanoscale cuboctahedral cages through weak interactions between their hydrophobic alkyl/aromatic surfaces. In general, amorphous solids were obtained via rapid precipitation from the mother liquor upon methanol addition, while crystalline solids could be obtained only following further chloroform and pyridine additions. The structure of the materials is confirmed via gas sorption and spectroscopic methods, while powder X-ray diffraction and transmission electron microscopy are used to determine the nature of these bulk solids.

Transient intermediate in the formation of an amorphous metal-organic framework

Amorphous metal-organic frameworks are rarely formed via direct synthesis. Our limited understanding of their atomic assembly in solution prevents full exploitation of their unique structural complexity. Here, we use in situ synchrotron X-ray absorption spectroscopy with sub-second time resolution to probe the formation of the amorphous Fe-BTC framework. Using a combination of spectral fingerprinting, linear combination analysis, and principal component analysis coupled with kinetic analyses, we reveal a multi-stage formation mechanism that, crucially, proceeds via the generation of a transient intermediate species.

In Situ Gelled Covalent Organic Frameworks Electrolyte with Long-Range Interconnected Skeletons for Superior Ionic Conductivity

Covalent organic frameworks (COFs) present an ideal platform for ion transport owing to their tunable and ordered nanochannels at the single-digit scale. However, achieving superior COF-based electrolytes remains challenging because of the mismatch between the intricate synthesis processes of COFs and the battery preparation environment, which makes it difficult to build continuous ion channels and low-impedance electrochemical interfaces for devices. Here, we present an in situ gelation method to produce COF gel electrolytes (CGEs) within liquid carbonate electrolyte, integrating COF synthesis with their applicability in batteries. This method leads to long-range interconnected and highly crystalline skeletons of COFs from a robust precoordination structure between lithium salts of liquid electrolyte and building blocks. By incorporating the lithium affinity groups in the COFs, the developed CGEs show a remarkable 3-fold enhancement in ionic conductivity, reaching up to 10.5 mS cm-1 compared to the corresponding liquid carbonate electrolytes. Furthermore, the CGEs exhibit a low activation energy of 0.068 eV, ensuring efficient ion transport, while demonstrating dendrite-free lithium deposition even after prolonged testing periods exceeding 1800 h. These CGEs exhibit excellent rate performance (reversible capacity up to 101 mAh g-1 at a current density of 3C, 1C = 170 mAh g-1) in Li-LiFePO4 coin cells and reversible cycling under extreme conditions (reversible capacity up to 158 mAh g-1 under folding state at 0.1C) in pouch cells. Importantly, our novel methodology extends beyond lithium-ion systems, as it can also be applied to the synthesis of CGEs utilizing potassium, magnesium, zinc, sodium, and calcium ions.

Pore-Networked Soft Materials Based on Metal-Organic Polyhedra

ConspectusThe last two decades have witnessed a tremendous development of crystalline microporous adsorbents in a wide range of applications including molecular adsorption, storage and separation, purification, as well as catalysis. The main players as porous materials that have contributed to the developments are extended molecular frameworks (e.g., metal-organic frameworks, MOFs; covalent-organic frameworks, COFs) or discrete porous molecules (e.g., metal-organic cages, MOCs; porous organic cages, POCs) thanks to the high degrees of freedom in their structural designability and tunability. To overcome the processability issue originating from their powder forms after synthesis, one main strategy is to hybridize the microporous adsorbents as pore-containing fillers with solvents or polymers as processable matrices to produce porous soft materials, such as porous liquids, gels/aerogels, and mixed-matrix membranes, depending on the form of matrix used. Nevertheless, the fabrication of "ideal" hybrid materials relies on the homogeneous distribution of the pore-containing fillers within the matrices. It is still challenging to find a versatile way to solve the aggregation issues of fillers and their insufficient interaction with the matrices, which are concerned with inhibiting the translation of the distinctive properties of microporous adsorbents into the obtained hybrid soft materials.Herein, we describe a new bottom-up approach for the fabrication of "pore-networked soft materials" based on the concept of directly interconnecting the pore-containing fillers into a continuous pore network within the matrices. The advantages of the pore-networking strategy lie in two main aspects: (i) the elimination of the need to struggle with the aggregation issue of fillers due to their overall interconnection throughout the matrices; (ii) the generation of continuous pore networks that guarantee the efficient molecular mass transfer in the materials. In this Account, we summarize our state-of-the-art progress of pore-networked soft materials based on the use of MOCs, alternatively called metal-organic polyhedra (MOPs) herein, as pore units for the pore network construction. The good solubility of MOPs in organic solvents allows them to be feasibly processed in solution, wherein the coordination of MOPs with organic linkers leads to the formation of linked MOP gels featuring not only intrinsic MOP cavities but also tunable extrinsic porosities generated between linked MOPs through the control of MOP/linker structures and network connectivity. Furthermore, the matrix of the linked MOP network, here referred to as the continuous phase with respect to the entire porous MOP network, is not limited to the solvents. We anticipate that the implementation of air, liquids, and polymers as the matrices could result in different forms of pore-networked soft materials like aerogels, foams, gels, monoliths, and membranes. For instance, we demonstrate the fabrication of linked MOP aerogel and permanently porous gel with their potential applications on selective CO2 photoreduction and gas sorption, respectively. We believe that the pore-network strategies will advance the development of porous soft materials featuring unique advantages and properties beyond the current hybrid systems.

Dynamic Cages-Towards Nanostructured Smart Materials

The interest in capsular assemblies such as dynamic organic and coordination cages has blossomed over the last decade. Given their chemical and structural variability, these systems have found applications in diverse fields of research, including energy conversion and storage, catalysis, separation, molecular recognition, and live-cell imaging. In the exploration of the potential of these discrete architectures, they are increasingly being employed in the formation of more complex systems and smart materials. This Review highlights the most promising pathways to overcome common drawbacks of cage systems (stability, recovery) and discusses the most promising strategies for their hybridization with systems featuring various dimensionalities. Following the description of the most recent advances in the fabrication of zero to three-dimensional cage-based systems, this Review will provide the reader with the structure-dependent relationship between the employed cages and the properties of the materials.

Porous supramolecular gels produced by reversible self-gelation of ruthenium-based metal-organic polyhedra

Supramolecular gels based on metal-organic polyhedra (MOPs) represent a versatile platform to access processable soft materials with controlled porosity. Herein, we report a self-gelation approach that allows the reversible assembly of a novel Ru-based MOP in the form of colloidal gels. The presence of cationic mixed-valence [Ru2(COO)4]+ paddlewheel units allows for modification of the MOP charge via acid/base treatment, and therefore, its solubility. This feature enables control over supramolecular interactions, making it possible to reversibly force MOP aggregation to form nanoparticles, which further assemble to form a colloidal gel network. The gelation process was thoroughly investigated by time-resolved ζ-potential, pH, and dynamic light scattering measurements. This strategy leads to the evolution of hierarchically porous aerogel from individual MOP molecules without using any additional component. Furthermore, we demonstrate that the simplicity of this method can be exploited for the obtention of MOP-based gels through a one-pot synthetic approach starting from MOP precursors.

Highly efficient and recyclable monolithic bioreactor for interfacial enzyme catalysis

HYPOTHESIS

Biocatalysts are key to the realization of all bioconversions in nature. However, the difficulty of combining the biocatalyst and other chemicals in one system limits their application in artificial reaction systems. Although some effort, such as Pickering interfacial catalysis and enzyme-immobilized microchannel reactors, have addressed this challenge an effective method to combine chemical substrates and biocatalysts in a highly efficient and re-usable monolith system is still to be developed.

EXPERIMENTS

A repeated batch-type biphasic interfacial biocatalysis microreactor was developed using enzyme-loaded polymersomes in the void surface of porous monoliths. Polymersomes, loaded with Candida antarctica Lipase B (CALB), are fabricated by self-assembly of the copolymer PEO-b-P(St-co-TMI) and used to stabilize oil-in-water (o/w) Pickering emulsions as a template to prepare monoliths. By adding monomer and Tween 85 to the continuous phase, controllable open-cell monoliths are prepared to inlay CALB-loaded polymersomes in the pore walls.

FINDINGS

The microreactor is proven to be highly effective and recyclable when a substrate flows through it, which offers superior benefits of absolute separation to a pure product and no enzyme loss. The relative enzyme activity is constantly maintained above 93% in 15 cycles. The enzyme is constantly present in the microenvironment of the PBS buffer ensuring its immunity to inactivation and facilitating its recycling.

Water-Soluble Ionic Metal-Organic Polyhedra as a Versatile Platform for Enzyme Bio-immobilization

Metal-organic polyhedra (MOPs) can act as elementary structural units for the design of modular porous materials; however, their association with biological systems remains greatly restricted by their typically low stabilities and solubilities in water. Herein, we describe the preparation of novel MOPs bearing either anionic or cationic groups and exhibiting a high affinity for proteins. Simple mixing of the protein bovine serum albumin (BSA) and ionic MOP aqueous solutions resulted in the spontaneous formation of MOP-protein assemblies, in a colloidal state or as solid precipitates depending on the initial mixing ratio. The versatility of the method was further illustrated using two enzymes, catalase and cytochrome c, with different sizes and isoelectric points (pI's) below and above 7. This mode of assembly led to the high retention of catalytic activity and enabled recyclability. Furthermore, the co-immobilization of cytochrome c with highly charged MOPs resulted in a substantial 44-fold increase of its catalytic activity.

Control of Extrinsic Porosities in Linked Metal-Organic Polyhedra Gels by Imparting Coordination-Driven Self-Assembly with Electrostatic Repulsion

The linkage of metal-organic polyhedra (MOPs) to synthesize porous soft materials is one of the promising strategies to combine processability with permanent porosity. Compared to the defined internal cavity of MOPs, it is still difficult to control the extrinsic porosities generated between crosslinked MOPs because of their random arrangements in the networks. Herein, we report a method to form linked MOP gels with controllable extrinsic porosities by introducing negative charges on the surface of MOPs that facilitates electrostatic repulsion between them. A hydrophilic rhodium-based cuboctahedral MOP (OHRhMOP) with 24 hydroxyl groups on its outer periphery can be controllably deprotonated to impart the MOP with tunable electrostatic repulsion in solution. This electrostatic repulsion between MOPs stabilizes the kinetically trapped state, in which an MOP is coordinated with various bisimidazole linkers in a monodentate fashion at a controllable linker/MOP ratio. Heating of the kinetically trapped molecules leads to the formation of gels with similar colloidal networks but different extrinsic porosities. This strategy allows us to design the molecular-level networks and the resulting porosities even in the amorphous state.

Assembling metal-organic cages as porous materials

There is growing interest in metal-organic cages (MOCs) as porous materials owing to their processability in solution. The discrete molecular character and surface features of MOCs have a direct impact on the interactions between cages, enabling the final physical state of the materials to be tuned. In this tutorial review, we discuss how to use MOCs as core building units, highlighting the role played by surface functionalisation of MOCs in leading to porous materials in a range of states covering crystalline solids, soft matter, liquids and composites. We finish by providing an outlook on the opportunities for this work to serve as a foundation for the development of increasingly complex functional porous materials structured over various length scales.

Metal-Organic Polyhedra as Building Blocks for Porous Extended Networks

Metal-organic polyhedra (MOPs) are a subclass of coordination cages that can adsorb and host species in solution and are permanently porous in solid-state. These characteristics, together with the recent development of their orthogonal surface chemistry and the assembly of more stable cages, have awakened the latent potential of MOPs to be used as building blocks for the synthesis of extended porous networks. This review article focuses on exploring the key developments that make the extension of MOPs possible, highlighting the most remarkable examples of MOP-based soft materials and crystalline extended frameworks. Finally, the article ventures to offer future perspectives on the exploitation of MOPs in fields that still remain ripe toward the use of such unorthodox molecular porous platforms.

Hypercrosslinked Polymer Gels as a Synthetic Hybridization Platform for Designing Versatile Molecular Separators

Hypercrosslinked polymers (HCPs), amorphous microporous three-dimensional networks based on covalent linkage of organic building blocks, are a promising class of materials due to their high surface area and easy functionalization; however, this type of material lacks processability due to its network rigidity based on covalent crosslinking. Indeed, the development of strategies to improve its solution processability for broader applications remains challenging. Although HCPs have similar three-dimensionally crosslinked networks to polymer gels, HCPs usually do not form gels but insoluble powders. Herein, we report the synthesis of HCP gels from a thermally induced polymerization of a tetrahedral monomer, which undergoes consecutive solubilization, covalent bond formation, colloidal formation, followed by their aggregation and percolation to yield a hierarchically porous network. The resulting gels feature concentration-dependent hierarchical porosities and mechanical stiffness. Furthermore, these HCP gels can be used as a platform to achieve molecular-level hybridization with a two-dimensional polymer during the HCP gel formation. This method provides functional gels and corresponding aerogels with the enhancement of porosities and mechanical stiffness. Used in column- and membrane-based molecular separation systems, the hybrid gels exhibited a separation of water contaminants with the efficiency of 97.9 and 98.6% for methylene blue and KMnO₄, respectively. This result demonstrated the potentials of the HCP gels and their hybrid derivatives in separation systems requiring macroscopic scaffolds with hierarchical porosity.

Rhodium-Based Metal-Organic Polyhedra Assemblies for Selective CO2 Photoreduction

Heterogenization of molecular catalysts via their immobilization within extended structures often results in a lowering of their catalytic properties due to a change in their coordination sphere. Metal-organic polyhedra (MOP) are an emerging class of well-defined hybrid compounds with a high number of accessible metal sites organized around an inner cavity, making them appealing candidates for catalytic applications. Here, we demonstrate a design strategy that enhances the catalytic properties of dirhodium paddlewheels heterogenized within MOP (Rh-MOP) and their three-dimensional assembled supramolecular structures, which proved to be very efficient catalysts for the selective photochemical reduction of carbon dioxide to formic acid. Surprisingly, the catalytic activity per Rh atom is higher in the supramolecular structures than in its molecular sub-unit Rh-MOP or in the Rh-metal-organic framework (Rh-MOF) and yields turnover frequencies of up to 60 h^-1 and production rates of approx. 76 mmole formic acid per gram of the catalyst per hour, unprecedented in heterogeneous photocatalysis. The enhanced catalytic activity is investigated by X-ray photoelectron spectroscopy and electrochemical characterization, showing that self-assembly into supramolecular polymers increases the electron density on the active site, making the overall reaction thermodynamically more favorable. The catalyst can be recycled without loss of activity and with no change of its molecular structure as shown by pair distribution function analysis. These results demonstrate the high potential of MOP as catalysts for the photoreduction of CO₂ and open a new perspective for the electronic design of discrete molecular architectures with accessible metal sites for the production of solar fuels.

Hierarchical porous metal–organic gels and derived materials: from fundamentals to potential applications

This review summarizes recent progress in the development and applications of metal–organic gels (MOGs) and their hybrids and derivatives dividing them into subclasses and discussing their synthesis, design and structure–property relationship.

Multiscale structural control of linked metal-organic polyhedra gel by aging-induced linkage-reorganization

Assembly of permanently porous metal-organic polyhedra/cages (MOPs) with bifunctional linkers leads to soft supramolecular networks featuring both porosity and processability. However, the amorphous nature of such soft materials complicates their characterization and thus limits rational structural control. Here we demonstrate that aging is an effective strategy to control the hierarchical network of supramolecular gels, which are assembled from organic ligands as linkers and MOPs as junctions. Normally, the initial gel formation by rapid gelation leads to a kinetically trapped structure with low controllability. Through a controlled post-synthetic aging process, we show that it is possible to tune the network of the linked MOP gel over multiple length scales. This process allows control on the molecular-scale rearrangement of interlinking MOPs, mesoscale fusion of colloidal particles and macroscale densification of the whole colloidal network. In this work we elucidate the relationships between the gel properties, such as porosity and rheology, and their hierarchical structures, which suggest that porosity measurement of the dried gels can be used as a powerful tool to characterize the microscale structural transition of their corresponding gels. This aging strategy can be applied in other supramolecular polymer systems particularly containing kinetically controlled structures and shows an opportunity to engineer the structure and the permanent porosity of amorphous materials for further applications.

Porous Colloidal Hydrogels Formed by Coordination-Driven Self-Assembly of Charged Metal-Organic Polyhedra

Introduction of porosity into supramolecular gels endows soft materials with functionalities for molecular encapsulation, release, separation and conversion. Metal-organic polyhedra (MOPs), discrete coordination cages containing an internal cavity, have recently been employed as building blocks to construct polymeric gel networks with potential porosity. However, most of the materials can only be synthesized in organic solvents, and the examples of porous, MOP-based hydrogels are scarce. Here, we demonstrate the fabrication of porous hydrogels based on [Rh₂ (OH-bdc)₂ ]₁₂ , a rhodium-based MOP containing hydroxyl groups on its periphery (OH-bdc=5-hydroxy-1,3-benzenedicarboxylate). By simply deprotonating [Rh₂ (OH-bdc)₂ ]₁₂ with the base NaOH, the supramolecular polymerization between MOPs and organic linkers can be induced in the aqueous solution, leading to the kinetically controllable formation of hydrogels with hierarchical colloidal networks. When heating the deprotonated MOP, Na_x [Rh₂₄ (O-bdc)_x (OH-bdc)_24-x ], to induce gelation, the MOP was found to partially decompose, affecting the mechanical property of the resulting gels. By applying a post-synthetic deprotonation strategy, we show that the deprotonation degree of the MOP can be altered after the gel formation without serious decomposition of the MOPs. Gas sorption measurements confirmed the permanent porosity of the corresponding aerogels obtained from these MOP-based hydrogels, showing potentials for applications in gas sorption and catalysis.

Light-induced assembly and disassembly of polymers with Pd n L2n -type network junctions

Polymers containing Pd n L2n complexes as network junctions were obtained by reaction of poly(ethylene glycol)-linked N-donor ligands with Pd2+. The addition of a metastable state photoacid renders the networks light sensitive, and gel-sol transitions can be achieved by irradiation with light. The inverse process, a light-induced sol-gel transition, was realized by using a molecularly defined Pd complex as an acid-sensitive reservoir for Pd2+. Upon irradiation, Pd2+ ions are released, allowing the formation of an acid-resistant polymer network. Both the gel-sol and the sol-gel transitions are reversed in the dark.

Spatiotemporal Control of Supramolecular Polymerization and Gelation of Metal-Organic Polyhedra

In coordination-based supramolecular materials such as metallogels, simultaneous temporal and spatial control of their assembly remains challenging. Here, we demonstrate that the combination of light with acids as stimuli allows for the spatiotemporal control over the architectures, mechanical properties, and shape of porous soft materials based on metal-organic polyhedra (MOPs). First, we show that the formation of a colloidal gel network from a preformed kinetically trapped MOP solution can be triggered upon addition of trifluoroacetic acid (TFA) and that acid concentration determines the reaction kinetics. As determined by time-resolved dynamic light scattering, UV-vis absorption, and ¹H NMR spectroscopies and rheology measurements, the consequences of the increase in acid concentration are (i) an increase in the cross-linking between MOPs; (ii) a growth in the size of the colloidal particles forming the gel network; (iii) an increase in the density of the colloidal network; and (iv) a decrease in the ductility and stiffness of the resulting gel. We then demonstrate that irradiation of a dispersed photoacid generator, pyranine, allows the spatiotemporal control of the gel formation by locally triggering the self-assembly process. Using this methodology, we show that the gel can be patterned into a desired shape. Such precise positioning of the assembled structures, combined with the stable and permanent porosity of MOPs, could allow their integration into devices for applications such as sensing, separation, catalysis, or drug release.

Directional asymmetry over multiple length scales in reticular porous materials

In nature and synthetic materials, asymmetry is a useful tool to create complex and functional systems constructed from a limited number of building blocks. Reticular chemistry has allowed the synthesis of a wide range of discrete and extended structures, from which modularity permits the controlled assembly of their constituents to generate asymmetric configurations of pores or architectures. In this perspective, we present the different strategies to impart directional asymmetry over nano/meso/macroscopic length scales in porous materials and the resulting novel properties and applications.

Design strategies for the controlled assembly of discrete and extended reticular materials with asymmetric configurations of pores or architectures.

Porous Functionally Graded Plates: An Assessment of the Influence of Shear Correction Factor on Static Behavior

The known multifunctional characteristic of porous graded materials makes them very attractive in a number of diversified application fields, which simultaneously poses the need to deepen research efforts in this broad field. The study of functionally graded porous materials is a research topic of interest, particularly concerning the modeling of porosity distributions and the corresponding estimations of their material properties—in both real situations and from a material modeling perspective. This work aims to assess the influence of different porosity distribution approaches on the shear correction factor, used in the context of the first-order shear deformation theory, which in turn may introduce significant effects in a structure’s behavior. To this purpose, we evaluated porous functionally graded plates with varying composition through their thickness. The bending behavior of these plates was studied using the finite element method with two quadrilateral plate element models. Verification studies were performed to assess the representativeness of the developed and implemented models, namely, considering an alternative higher-order model also employed for this specific purpose. Comparative analyses were developed to assess how porosity distributions influence the shear correction factor, and ultimately the static behavior, of the plates.